1. Field of the Invention
The invention relates in general to a control method of an optical disc drive, and more particularly to a method for determining a phase difference that causes a tracking error displacement in track control of an optical disc drive.
2. Description of the Related Art
By focusing projected beam spots to an optical disc, an optical disc drive forms control signals of focusing error signal and tracking error signal through differences in the amounts of reflected light beams. With the control signals, the optical disc drive keeps the beam spots remain focused at the optical disc and move along data tracks to read data from/write data to the optical disc.
FIG. 1 shows a functional block diagram of an optical disc drive generating a tracking error; FIG. 2 shows a schematic diagram of a tracking error. When the conventional optical disc drive performs track control via differential push-pull (DPP), a pickup head focuses laser beams to a main light beam 1a and two secondary light beams 1b and 1c, which are respectively projected to a data groove 2 and two lands 3. The projected light beams are reflected by an optical disc into reflected beam spots 4a, 4b and 4c, which are then respectively projected to a main optical transducer 5a and two secondary optical transducers 5b and 5c. The optical transducers 5a, 5b and 5c are respectively divided into two same-sized sub-units E and F, and convert light flux at the reflected beam spots 4a, 4b and 4c into corresponding electric signals. The electric signal E1−F1 of two sub-units of the main optical transducer 5a forms a main push-pull (MPP) signal. The electric signals [(E2−F2)+(E3−F3)] of the two sub-units of the two secondary transducers 5b and 5c are adjusted by a gain to a size substantially the same as the MPP signal to form a secondary push-pull (SPP) signal. The SPP signal is subtracted from the MPP signal (MPP−SPP) to form the tracking error signal, which serves as a control signal for the tracking of the optical disc drive.
An optimal projection angle θ between the main and secondary beams projected from the pickup head and the data groove is generally designed to render a 180-degree phase difference between the MPP signal and the SPP signal, so that the tracking error signal formed by (MPP−SPP) is given a maximum value to obtain an ideal tracking error signal that facilitates the control of the main beam 1a along of data groove 2, thereby correctly reading marks in the data groove 2. However, due to factors such as poor manufacturing, unsatisfactory quality, eccentric optical discs and relative oppositions of optical discs, an angle between the main and secondary beams and the data groove 2 may not be the predetermined optimal angle θ, such that a phase difference between the MPP signal and the SPP signal is not the predetermined phase difference either. As indicated by a dotted line in FIG. 2, the tracking error signal formed by (MPP−SPP) is attenuated as a result. To overcome the issue of phase difference, a phase difference detection circuit is proposed by the prior art for detecting the phase difference. However, the phase difference detection circuit increases circuit complexity and manufacturing costs.
Therefore, there is a need for an improved solution for determining the phase difference of a tracking error signal to obviate the abovementioned problems associated with the prior art.